[Paper Review] A new view of the giant molecular cloud M16 (Eagle Nebula) in 12CO J=1-0 and 2-1 transitions with NANTEN2

Atsushi Nishimura, Jean C Costes+12 more|arXiv (Cornell University)|Jun 19, 2017

Astrophysics and Star Formation Studies2 references8 citations

TL;DR

This study presents new NANTEN2 CO (J=1-0 and J=2-1) observations revealing a 1.3×10⁵ M☉ giant molecular cloud (GMC) in M16 (Eagle Nebula), elongated 35 pc vertically from the Galactic plane. The authors propose that a collision between blue- and red-shifted velocity components at ~10 km s⁻¹ triggered O-star formation in M16 and N19, with ionization cavities and complementary gas distributions supporting this scenario.

ABSTRACT

M16, the Eagle Nebula, is an outstanding HII region where extensive high-mass star formation is taking place in the Sagittarius Arm, and hosts the remarkable "pillars" observed with HST. We made new CO observations of the region in the 12CO J=1--0 and J=2--1 transitions with NANTEN2. These observations revealed for the first time that a giant molecular cloud of $\sim 1.3 imes 10^5$ \Msun \ is associated with M16, which is elongated vertically to the Galactic plane over 35 pc at a distance of 1.8 kpc. We found a cavity of the molecular gas of $\sim 10$ pc diameter toward the heart of M16 at \lbeq (16.95\degree, 0.85\degree), where more than 10 O-type stars and $\sim 400$ stars are associated, in addition to a close-by molecular cavity toward a Spitzer bubble N19 at \lbeq (17.06\degree, 1.0\degree). We found three velocity components which show spatially complementary distribution in the entire M16 giant molecular cloud (GMC) including NGC6611 and N19, suggesting collisional interaction between them. Based on the above results we frame a hypothesis that collision between the red-shifted and blue-shifted components at a relative of $\sim 10$ \kms \ triggered formation of the O-type stars in the M16 GMC in the last 1-2 Myr. The collision is two fold in the sense that one of the collisional interactions is major toward the M16 cluster and the other toward N19 with a RCW120 type, the former triggered most of the O star formation with almost full ionization of the parent gas, and the latter an O star formation in N19.

Motivation & Objective

• To map the molecular gas structure and kinematics of the M16 giant molecular cloud (GMC) using high-resolution CO line observations.
• To investigate the dynamical interaction between molecular gas components and their relation to massive star formation in M16 and associated regions.
• To test the hypothesis that cloud-cloud collision triggered O-star formation in M16 and the Spitzer bubble N19.
• To determine the mass, size, and kinematic structure of the GMC to assess its role in high-mass star formation.
• To examine the effects of ionization from O stars on the surrounding molecular gas, including cavity formation and gas depletion.

Proposed method

• Conducted large-scale CO J=1–0 and J=2–1 line observations of M16 and N19 using the NANTEN2 telescope at the Nobeyama Radio Observatory.
• Mapped the spatial and velocity distribution of molecular gas across the M16 GMC, identifying three distinct velocity components: 14.8–18.8 km s⁻¹ (blue), 22.8–28.8 km s⁻¹ (main), and 28.8–30.8 km s⁻¹ (red).
• Estimated the total mass of the GMC using the X_CO factor applied to the ¹²CO J=1–0 line emission, yielding 1.3×10⁵ M☉.
• Analyzed spatial complementarity between velocity components to infer dynamical interaction, particularly between blue- and red-shifted gas.
• Identified intensity depressions in molecular emission toward the M16 core and N19, interpreted as ionization cavities due to UV radiation from O stars.
• Used kinematic and spatial correlations between gas components and stellar clusters to infer collisional triggering of star formation.

Experimental results

Research questions

• RQ1 What is the spatial and kinematic structure of the giant molecular cloud associated with M16?
• RQ2 How do the velocity components of molecular gas in M16 correlate spatially, and what does this imply about dynamical interactions?
• RQ3 What is the role of cloud-cloud collision in triggering O-star formation in M16 and N19?
• RQ4 To what extent has the molecular gas in M16 been ionized by the 11 O stars in NGC6611?
• RQ5 How do the observed ionization cavities and pillar structures relate to the timing and mechanism of massive star formation?

Key findings

• The M16 GMC spans 20 pc × 35 pc in Galactic longitude and latitude, with a total mass of 1.3×10⁵ M☉ derived from ¹²CO J=1–0 emission.
• Three distinct velocity components were identified: blue-shifted (14.8–18.8 km s⁻¹), main (22.8–28.8 km s⁻¹), and red-shifted (28.8–30.8 km s⁻¹), with the main and red-shifted components showing complementary spatial distribution.
• A molecular cavity of ~10 pc diameter is observed toward the M16 core at (l, b) = (16.95°, 0.85°), consistent with ionization by 11 O stars in NGC6611.
• A second cavity is found toward the Spitzer bubble N19 at (l, b) = (17.06°, 1.0°), associated with an O9 star, indicating ongoing but less intense ionization.
• The blue- and red-shifted components show spatial complementarity across the entire GMC, suggesting a collisional interaction at ~10 km s⁻¹ relative velocity.
• The collision timescale is estimated at ~3×10⁵ yr, consistent with the age of O-star formation in M16 (~1–2 Myr), supporting the collision-triggering hypothesis.